First of all your bad guy needs to have a deep rooted desire and need for something. Your antagonist will of course get in the way of that and that is why they will fight. Your bad guy wants something so badly he will do anything in order to get it.
Your bad guy should have some sort of odd habit or quirk. A physical variation can work as well but a behavioral quirkiness is much more interesting. This gives him something that stands out in a crowd or that we can see coming from a distance.
Your antagonist has to be human. No matter how bad your bad guy is he still needs to have a soft side. It could be a weakness for kids.
Make your bad guys smart. A dumb bad guy who fumbles around is fine, but a really clever villian is much more interesting and will take your story to a new level. Think "Hannibal Lector." He is a brilliant man who does unspeakable things but is still compelling.